Recently, significant progress has been made in the polymer industry, which has resulted in use of varied and diverse polymer materials over a wide range. In particular, with the enhancement in function and performance of industrial products, development of a more excellent polymer material is proceeding.
Among such materials, an epoxy resin has wide industrial applications as a thermosetting resin or other reactive resins and has been studied and developed from various aspects. The epoxy resin that is most widely used in the industry at present is a bisphenol A-type epoxy resin produced by the reaction of bisphenol A and epichlorohydrin.
This resin includes a wide range of liquid to solid products, and is also excellent in reactivity, chemical resistance, toughness, adhesion, heat resistance and the like, and therefore it finds extensive uses, for example, in civil engineering, architecture, coatings and adhesives. However, since the bisphenol A-type epoxy resin is obtained by the reaction of bisphenol A and epichlorohydrin, tens of ppm to 100 ppm of chlorine is contained in the resin, and this brings about a problem, such as impairment of electrical characteristics of an electric component. Furthermore, with the recent reduction in size and weight of an electrical component, a flexible wiring board becomes increasingly popular, and flexibility is also required of the insulating resin itself so as to reduce the thickness and weight. Accordingly, an epoxy resin free from chlorine and balanced in the electrical characteristics, heat resistance and flexibility is demanded.
An alicyclic epoxy resin is known as a chlorine-free resin, and a compound represented by the following formula (e) or (f):
is industrially produced and used as a raw material of the alicyclic epoxy resin. Such a material has a high glass transition temperature and excellent heat resistance but is disadvantageously low in flexibility.
As for other epoxy resins containing reduced residual halogen, there have been proposed, for example, an epoxy resin obtained by ring-opening polymerizing 4-vinyl-cyclohexane oxide and then epoxidizing the vinyl group (see, Patent Document 1 below), a resin produced by epoxidizing a compound obtained through alternating copolymerization of 4-vinyl-cyclohexane oxide and an acid anhydride (see, Patent Document 2 below), an epoxy resin composition obtained by polymerizing a methacrylic acid ester of epoxycyclohexanemethanol with another methacrylic or acrylic acid ester (see, Patent Document 3 below), and an epoxy resin composition obtained by polymerizing allyl 3,4-epoxycyclohexane-1-carboxylate with an allyl ester, a vinyl ester, a vinyl ether, a (meth)acrylic acid ester or the like (see, Patent Document 4 below). However, still higher performance is demanded in electrical characteristics, heat resistance and flexibility.
On the other hand, a thermosetting resin, such as unsaturated polyester resins and epoxy (meth)acrylate resins, is being widely used, for example, as a base material of FRP (fiber-reinforced plastic) for an electronic material, a building material, a transportation device, industrial equipment and materials and the like, or for a casting mold, a coating material, an adhesive, a resin concrete, a decorative sheet and the like.
The unsaturated polyester resin is a viscous liquid resin generally obtained by polycondensing an alcohol component composed of a polyhydric alcohol and an acid component composed of α,β-unsaturated polyvalent carboxylic acids and saturated polyvalent carboxylic acids or aromatic polyvalent carboxylic acids, and blending the resulting unsaturated polyester with a radical polymerizable monomer, such as styrene. By changing the kinds and amounts of the polyhydric alcohol as well as the acid component composed of α,β-unsaturated polyvalent carboxylic acids and saturated polyvalent carboxylic acids or aromatic polyvalent carboxylic acids used for the production of an unsaturated polyester, an unsaturated polyester resin composition having physical properties suitable for various intended uses or being moldable by a molding method suitable for the intended use can be produced.
An epoxy (meth)acrylate resin is derived from a polyhydric phenol-type epoxy resin, such as bisphenol-type epoxy resins and novolak-type epoxy resins, and (meth)acrylic acid, and is known as a resin with excellent moldability in view of curability, workability and the like, and usually, such a resin is also widely used by blending a radical polymerizable crosslinking agent, such as styrene.
However, this resin system contains approximately from 30 to 60 mass % of styrene, which is a radical polymerizable monomer. Thus, in an open-mold molding method, such as hand lay-up molding and spray-up molding, styrene contained in the resin often volatilizes during FRP shaping to worsen the molding working environment. In recent years, for example, the law of PRTR (Pollutant Release and Transfer Register) has been implemented, and regulations on discharge of chemical substances are tightened. Under such conditions, styrene contained in the resin above comes under a substance to be regulated. It is required of course in open-mold molding but also in other molding methods to reduce the amount of volatilized styrene so as to meet the regulations or to improve the molding working environment.
As for a method for reducing the content of a styrene monomer, there is a method of keeping low the molecular weight of a thermosetting resin, such as unsaturated polyester resins and epoxy acrylate resins, to lower the viscosity and thereby decrease the blending amount of the monomer (for example, decrease the amount of styrene).
In the case of an unsaturated polyester resin, the method for reducing the molecular weight generally includes a method of controlling the reaction to keep the molecular weight low, a method of capping the molecular terminal by modification with dicyclopentadiene to keep the molecular weight low (see, for example, Patent Documents 5 and 6 below), and a method of replacing a part of the polyhydric alcohol with a monoalcohol to cap the molecular terminal and thereby keep the molecular weight low (see, for example, Patent Document 7 below). In these methods, the absolute amount of styrene contained in the unsaturated polyester resin can be reduced to about 30 mass %, and therefore instability in the effect of suppressing styrene and decrease of the secondary adhesion, which are observed when utilizing a paraffin wax-based additive, do not occur, and therefore a stable effect of suppressing styrene can be obtained. Furthermore, since reduction in the viscosity can also be achieved by these methods, the molding workability, such as injection and filling in the RTM molding, can be improved and in the case of using the resin for a resin concrete, the amount of filler can be increased without impairing the moldability, such as fluidity and filling properties.
However, in the method of merely controlling the molecular weight, reduction in the molecular weight of the unsaturated polyester resin involves reduction in the mechanical properties of the resulting cured product, such as strength and elongation percentage, and moreover, the terminal group (hydroxyl group, carboxyl group) of the polyester increases, thereby resulting in the water resistance of the resulting cured product being greatly deteriorated.
In addition, in the case of a dicyclopentadiene-modified unsaturated polyester resin in which the molecular terminal is capped by using dicyclopentadiene so as to reduce the amount of a hydrophilic terminal group, the mechanical properties of the resulting cured product, such as strength and elongation percentage, are often impaired due to the chemical structure and decreased molecular weight of the dicyclopentadiene-modified unsaturated polyester resin.